AD9689-2000EBZ datasheet(61/134 Pages) AD

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Data Sheet

AD9689

Rev. A | Page 61 of 134

DDC GAIN STAGE

Each DDC contains an independently controlled gain stage.

The gain is selectable as either 0 dB or 6 dB. When mixing a real

input signal down to baseband, it is recommended that the user

enable the 6 dB of gain to recenter the dynamic range of the

signal within the full scale of the output bits.

When mixing a complex input signal down to baseband, the

mixer has already recentered the dynamic range of the signal

within the full scale of the output bits, and no additional gain is

necessary. However, the optional 6 dB gain compensates for low

signal strengths. The downsample by 2 portion of the HB1 FIR

filter is bypassed when using the complex to real conversion

stage. The TB1 filter does not have the 6 dB gain stage.

DDC COMPLEX TO REAL CONVERSION

Each DDC contains an independently controlled complex to

real conversion block. The complex to real conversion block

reuses the last filter (HB1 FIR) in the filtering stage along with

an fS/4 complex mixer to upconvert the signal. After upconverting

the signal, the Q portion of the complex mixer is no longer

needed and is dropped. The TB1 filter does not support

complex to real conversion.

Figure 126 shows a simplified block diagram of the complex to

real conversion.

LOW-PASS

FILTER

REAL

HB1 FIR

LOW-PASS

FILTER

HB1 FIR

COMPLEX TO

REAL ENABLE

90°

0°

–

COMPLEX TO REAL CONVERSION

GAIN STAGE

cos(ωt)

sin(ωt)

I/REAL

0dB

6dB

0dB

6dB

0dB

6dB

0dB

6dB

fS/4

Figure 126. Complex to Real Conversion Block

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